the name "fieldvalues" is non-specific. i realize it comes from the name in getaddress() but we should make it more clear what these are. email_address_fields perhaps?

Related: I don't this docstring adds meaningful value. naming the function and parameter right along with it being short code is sufficiently self explanatory for this internal function. get rid of the docstring.

See later comments on str(v) being bad, we can get rid of it here.

this loop is modifying the apparent list it is iterating over within the loop. that makes reasoning about its exact behavior hard. removing an item could mean you wind up skipping an item, appending an item could make the loop iterate over that. furthermore remove is O(n)... you're re-finding the item to remove to remove it. Also, this code modifys the passed in list in place before returning it.

A better code pattern for this is to build up a new list and return that. never modifying the input.

Something like:

accepted_values = []
for v in fieldvalues:
    s = v.replace('\\(', '').replace('\\)', '')
    if s.count('(') != s.count(')'):
        v = ('', '')
    accepted_values.append(v)

return accepted_values

parsed_email_address_tuples perhaps?

Add a comment explaining why only [ is not allowed. ideally with a link to the relevant RFC or similar.

same design comment as above, don't modify the list being iterated over and don't modify the argument in place.

add documentation mentioning that fieldvalues that could not be parsed may cause a ('', '') item to be returned in their place.

The existing code already calls str(v) here... this is a problem. It means the input can be anything. But we probably cannot change it in a patch release for a security fix.

To avoid propagating this mistake into more lines of code spread out, I suggest changing this function to do:

fieldvalues = [str(v) for v in fieldvalues]

on the first line and get rid of all subsequent str(v) calls on anything from fieldvalues in this function or in functions it calls.

get rid of this str(v)